Device for reducing acoustic oscillations originating from the rotation of a prismatic member in a cylindrical casing



5R m2 55a 6 A 55mm Nov. 11, 1969 c. H. ca. THUNHOLM 3, n I DEVICE FOR REDUCING ACOUSTIC OSCILLATIONS ORIGINATING FROM 3 THE ROTATION OF A PRISMATIC MEMBER IN RE. I A CYLINDRICAL CASING RQ$ Filed Sept. 25, 1968 l l I 6 i l l I a I 4 H6. 2

4 FIG. 3

INVENTOR. CARL mxw @usmv THUNHOLM BY ATTPRNEYS OR N 1/33! f U.S. Cl. 181- 33 4 Claims ABSTRACT OF THE DISCLOSURE A rotating mirror prism within a cylindrical drum with an opening for allowing radiation from the out-. side to strike the prism. One edge of the opening extends at an oblique angle to the generatrix lines of :the drum to prevent the air current produced by the rotation of the prism from striking all points on the edge simultaneously, thereby reducing the noise caused by the air current striking the edge.

BACKGROUND OF THE INVENTION The invention relates to a device for damping acoustic vibrations caused by the rotation of a prismatic body within a cylindrical drum which is arranged concentrically with the rotational axis of the prismatic body and the wall of which is provided with at least one opening arranged so that the sides of the prismatic abody pass by this opening in turn when the body rotates.

A prismatic body rotating within a drum is used for instance. in the'optical system of such cameras-for photographing with infrared light which are combined withan oscilloscope, on the picture screen of whichia thermal picture of the photographed object can be directly viewed. The prismatic body forms part of the system for scanning the photographed object, and its sides are formed by or coated with mirrors. The purpose of the lsurrounding drum is] to protect the rotating prism, but'it must of course be provided with an opening through which the incoming and reflected radiation can pass. Hitherto it has 'nited States Patent been common practice to give this opening a rectangular form with two opposite sides parallel to the axis of the 'mirror prism.

When the prismatic body rotates within a ,jrotational dium provided with such an opening, the air in the interstices between the inside of the drum and the sides of the prismatic body is caused to rotate along with the prismatic body, and this air current produces together with the edges of the opening and particularly that edge of the opening which meets the air current, a strong noise with a fundamental frequency dependent on the rotational speed of the prismatic body. This noise is most irritating and disturbing to the persons handling the apparatus and may also disturb the proper function of the apparatus.

SUMMARY OF THE INVENTION It is an object of the invention to reduce this drawback. According to the invention that edge of the opening, which is passed by the sides of the rotating prism in a direction from the opening to under the we 0 e rum, extends at an oblique angle to the generatrix lines of the wall of the drum. Hereby the compression of air which occurs when an edge of the prismatic body passes by the said edge of the opening, will not occur simul- 3,477,541 Patented Nov. 11, I969 ice DRAWING AND DETAILED DESCRIPTION The invention will be described more in particular in conjunction with the accompanying drawing.

FIG. 1 shows schematically and in top view a cylindrical drum and a rotating prismatic body arranged within the drum.

FIG. 2 shows the drum with the opening in side view, the opening having a form according to one embodiment of the invention.

FIG. 3 also shows the drum in side view and illustrates another form of the opening according to the invention. a.

In FIG. 1 the rotating prismatic body is designated 1 and in the illustratedexample the cross-section of the body has the shapeof a regular hexagon. The rotational axis of the body is designated 2, and its direction of rotation is indicated by an arrow in the figure. The prismatic body rotates within a cylindrical drum 3 the wall of which is provided with an opening 4.

In the embodiment shown in FIG. 2 the opening 4 is limited by two edges 5 and 6, of which edge 5 extends along a generatrix line on the drum, i.e. parallel to the axis of the drum and the rotating body, while thejlimiting edge 6 according to the invention extends at an oblique angle to the generatrix lines of the wall of the drum. The edge 6 extends continuously from a point at or adjacent to one end of thejdrum to a point at or adjacent "to the other end of the drum. The distance marked a in FIG. 2 is the difference between the largest and the least width of the opening 4, -;a' nd in order that the best effect shall be achieved this distance should be about equal to one half of a side inithe regular polygon which forms the cross section of the prismatic body 1.

In FIG. 3 that limiting edge of opening 4 which faces the rotational direction, is designated 16. In this efi'nbodiment the limiting edge has the shape of a brokeh, sawsidered as a combination of the embodiments shown in FIGS. 2 and 3.

The said limiting edge may also have other forms within the scope of the invention.

I claim:

1. A device for reducing acoustic oscillations caused by the rotation of a prismatic member within a casing of substantially cylindrical form which is arranged concentrically with the rotational axis of the prismatic memg'ber, and the wall of which is provided with'at least one '-;opening arranged so that the sides of the rotating "prismatic member in turn pass by the opening, characterized in that that edge of the opening, which is passed i by the sides of the rotating prismatic member in the direction from the opening to under the wall of the casing, extends at an oblique angle to the generatrix lines of the wall of the casing.

2. A device as claimed in claim 1, characterized in that said edge extends continuously from a point adjacent to one end of the casing to a point adjacent to the othe1 end of the casing.

3. A device as claimed in claim 1, characterized in that said edge is saw-tqoth-shaped.

-4. A device as claimed in claim 1 in which the prismatic member in a section at right angles to its rotational axis 4 FOREIGN PATENTS 3 has the form of a regular polygon, characterized in that the difierence between the largest and the least Width of 1 303 056 7/1962 France said opening is about equal to one half of a side in said 12/1934 Great 'Britain p s References Cited 5 ROBERT s. WARD, m, Primary Examiner UNITED STATES PATENTS US Cl. Re. 19,862 2/1936 Adsit 350-7 XR 

